I had to make a similar decision once. It was painful and heartbreaking. But once the decision was made, I felt a wave of comfort like I hadn't felt before or since. It carried me for a long time.

You say you can't fix this, but maybe you can? I have full faith and confidence in the reality of the atonement and the resurrection. I know that broken bodies will be made whole. By letting her go, you will fix all of her mortal issues and you will set her on her way to a life free of those pains and sufferings.

God bless you and your family.